Mick,
Ripped ya computer apart (take the sides or top off lol) and look for dust, my PC sits on the floor (on a rug) and the amount of dust and crap in there and on the fans was crazy. It cause my PC to shut down because of over heating i guess the CPU.
I grabbed bit of compressed gas (from dick smiths lol) cleaned the inside of the PC fans etc and all things are working well now touch on wood lol.
:)) hope all works out mate.
Woodsy